California Code, Public Utilities Code - PUC § 103363

(a) Any action or proceedings wherein the validity of the levy or collection of the retail transactions and use tax, or any proceedings in relation thereto, is contested, questioned, or denied shall be commenced not later than July 1, 1990; otherwise the retail transactions and use tax and all proceedings in relation thereto shall be held to be valid and in every respect legal, enforceable, and incontestable.

(b) Any action or proceedings wherein the validity of the issuance of any bonds, or any proceedings in relation thereto is contested, questioned, or denied shall be commenced not later than July 1, 1990, or, in the case of bonds authorized to be issued after that date, within six months from the date the bonds are authorized to be issued by the district; otherwise, in each case, the bonds and all proceedings in relation thereto shall be held to be valid and in every respect legal, enforceable, and incontestable.

(c) Nothing in this section precludes the validity of the retail transactions and use tax or of any bonds or any related proceedings from being established by an action brought under Chapter 9 (commencing with Section 860) of Title 10 of Part 2 of the Code of Civil Procedure or pursuant to any other procedure established by law, before the end of any period of time specified in subdivision (a) or (b).